The Earth's temperature has risen so sharply since 1970 compared to the
previous undulations that changes
in solar activity and the Earth's orbit cannot explain it.
It is true that there has been a slight
increase in solar heat during the last few centuries as the Earth
emerged from the Little Ice Age. But it doesn't nearly explain
the whole story.

By far the biggest factor causing the current, short-term global
warming, is the greenhouse
effect,
caused by the recent, accelerating accumulation of greenhouse gases
in the atmosphere.

The main greenhouse gases are water vapour, nitrous oxide, methane and
carbon dioxide. Carbon
dioxide is the most worrying one at the moment, although
methane could become a worrying one too.

Greenhouse
gases are necessary
as they keep the Earth from freezing over. For at least the past 650,000
years their levels have been very stable with carbon dioxide well below
300ppm (often below 200ppm), and methane below 800ppb. During the past 200 years however,
carbon dioxide has risen to almost 400ppm and methane has more than
doubled (Methane is not as worrying as CO2 because it doesn't stay in
the atmosphere for nearly as long as CO2).

The Antarctic ice sheet started to form when CO2 levels were 450ppm.
So, it will start melting
en
masse when we have reached that level. At the current
pace, we will reach that level in the next 40 years.

A seemingly small temperature rise will make huge differences. About
3-4 million years ago, the average temperatures of the Earth were about
2-3 degrees celsius higher than now. And the sea levels were 25 meters higher than our present
sea levels!

During the Eocene and Palaeocene warm period, the temperatures were 6
degrees celcius higher than our present temperatures. And there was a mass extinction.

It is clear that the Earth is fast reaching climatic conditions never experienced by human beings
before.

Note:
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